1. Field of the Invention
This invention relates to the field of liquid urea-sulfuric acid compositions, and particularly to such compositions having reduced corrosivity to carbon steel. The compositions are stable homogeneous solutions of urea-sulfuric acid reaction products of significantly reduced corrosivity toward carbon steels even at elevated temperatures and under flow conditions. Thus, they enable the use of such urea-sulfuric acid reaction products at elevated temperatures and/or under flow conditions during their manufacture, transport or use in carbon steel equipment.
2. Description of the Prior Art
Both urea and sulfuric acid are widely used for a variety of purposes in numerous industries as fertilizers, soil adjuvants, chemical treating agents, chemical precursors and reactants.
Previous investigators have observed that urea, sulfuric acid, and optionally water, can be reacted to form concentrated solutions of urea and sulfuric acid reaction products in which at least a portion of the urea is present as mono and/or diurea sulfate. However, they did not recognize that reaction products of different urea/sulfuric acid ratio exhibit markedly different corrosivity towards carbon steel, that the reaction products differ markedly from sulfuric acid with respect to their corrosivity and to their response to known sulfuric acid corrosion inhibitors, or that the corrosivity of all of the reaction products toward stainless steel could be markedly reduced by the addition of corrosion inhibiting amounts of cupric ion-containing compounds.
D. F. du Toit found that urea formed certain compounds with oxalic, acetic, hydrochloric, nitric and sulfuric acids, and that the resulting compounds were stable in contact with their solutions at 20.degree. C. Verslag Akad. Wetenschappen, 22, 573-4 (abstracted in Chemical Abstracts, 8, 2346, 1914).
L. H. Dalman expanded on du Toit's work by developing the phase relationships between the solid phase and saturated solutions at 10.degree. C. and 25.degree. C. "Ternary Systems of Urea and Acids. I. Urea, Nitric Acid and Water. II. Urea, Sulfuric Acid and Water. III. Urea, Oxalic Acid and Water"; JACS, 56, 549-53 (1934).
In the article "Adding Plant Nutrient Sulfur to Fertilizer," Sulfur Institute Bulletin No. 10 (1964), the Sulfur Institute discussed the addition of nutrient sulfur to fertilizers and mentioned that urea reacts with sulfuric acid to form two complexes of urea sulfate which are useful fertilizers.
Jones, U.S. Pat. No. 4,116,664 discloses what is referred to therein as a tortuous, multistage process of producing combinations of urea and sulfuric acid in which portions of the sulfuric acid are incrementally added to and reacted with the total amount of urea to be reacted in each of several stages until the total amount of sulfuric acid has been reacted with the urea. The resulting product is unstable and requires further processing. Jones preferably adds water later as required to obtain stability and the desired composition. He discloses that the reaction can be carried out at temperatures of 100.degree. to 200.degree. F. and that if the sulfuric acid is added to the total amount of urea at a rate which is too fast the temperature goes to about 200.degree. to 225.degree. F. and that a gas is emitted that causes changes in product characteristics such as solidification. The patent states that temperatures of 160.degree. to 200.degree. F. are preferred and that the products can be used as fertilizers.
A variety of compounds, including dialkylthioureas, such as diethylthiourea and dibutylthiourea, are known to reduce the corrosivity of sulfuric acid to carbon steels. Also, it is known that a wide variety of compounds, including cupric sulfate, reduce sulfuric acid corrosivity toward stainless steels.
Although these investigators disclosed several characteristics of urea-sulfuric acid combinations and methods of making those combinations, and that the products were useful soil adjuvants and/or fertilizers, they did not recognize that the corrosivity of the urea-sulfuric acid reaction products to carbon steel and the relationship of corrosivity to alkylthiourea concentration vary dramatically as a function of urea/sulfuric acid molar ratio, or that the corrosivity of some compositions to carbon steel can be dramatically reduced by the addition of corrosion inhibiting amounts of certain alkyl-substituted thiourea compounds. Nor did they recognize that the corrosivity of these compositions to stainless steel is reduced by cupric ion-containing compounds.
They were also not aware that the corrosion characteristics of the urea-sulfuric acid compositions of this invention differ markedly from those of sulfuric acid. For instance, all of the sulfuric acid inhibitors, with the exception of the alkylthioureas, have little or no beneficial effect on the urea-sulfuric acid compositions, and many significantly increase corrosion rate.
Furthermore, none of the inhibitors, including the thioureas, have any significant beneficial effect on compositions having urea/sulfuric acid molar ratios between 1 and 2. Corrosivity is relatively low within that composition range but is intolerably high with compositions having urea/sulfuric acid molar ratios above 2 or below 1. Thus, corrosivity increases dramatically as sulfuric acid concentration is either increased or decreased relative to urea concentration outside this range of molar ratios. Another anomalous characteristic of these compositions is that some, but not all of them are significantly more corrosive to stainless steel than they are to carbon steel. Thus, 10-0-0-19 corrodes AISI C-1010 carbon steel at a rate of 56 mils per year under static conditions at 150.degree. F. and corrodes AlSl type 316 stainless steel at 300 mils per year under identical conditions.
It is therefore one object of this invention to provide improved, liquid urea-sulfuric acid compositions.
It is another object of this invention to provide liquid, urea-sulfuric acid reaction product compositions which are relatively noncorrosive to carbon steel even at elevated temperatures or under fluid flow conditions.
It is another object of this invention to provide liquid, urea-sulfuric acid reaction product compositions of reduced corrosivity to both carbon steel and stainless steel.
Other objects, aspects and advantages of this invention will be apparent to one skilled in the art in view of the following disclosure, the drawings, and the appended claims.